The Information Tribunal has ruled that civil servants' advice to ministers on major planning decisions should be disclosed to anyone who asks for it once the decision is taken.

The ruling, obtained by the former Tory environment secretary Lord Baker, was hailed yesterday by Friends of the Earth as a "significant breakthrough which means that members of the public will now have a much better understanding of why a controversial major planning decision has been made".

The Department for Communities and Local Government - which took over responsibility for planning from the Office of the Deputy Prime Minister last year - has until June 29 to release the advice officials gave John Prescott when he was considering the application for the 50-storey residential Vauxhall Tower near Vauxhall Bridge overlooking the Thames in south London.

Mr Prescott approved the development in 2005 after the planning inspector concluded it was the wrong building for the wrong site and should not go ahead.

The ruling applies to planning applications "called in" for a decision by a minister, a process used in some of the biggest and most controversial projects. As environment secretary Kenneth Baker under Mrs Thatcher in the mid-1980s, Lord Baker approved the Channel tunnel and Canary Wharf developments, but he considered the decision to approve the Vauxhall Tower development "outrageous". "I'm not against tall buildings but tall buildings should be grouped. No other major city - not Madrid or Paris or Rome - would allow this sort of thing. I was appalled because the view from Westminster Bridge is a Unesco world heritage site."

He suspected that officials had recommended refusal of the application and applied to the information commissioner, Richard Thomas, after the department refused to release the civil servants' advice. Mr Thomas ruled that the submissions about the planning application should be released but with officials' advice and opinions edited out.

With the help, free of charge, of solicitor Dan Tench and barrister Hugh Tomlinson QC, Lord Baker appealed to the three-person information tribunal. The government argued that if officials knew their advice could be made public they would be constrained in expressing their views and would be more likely to communicate orally rather than on paper. Paul Hudson, chief planner for the department, told the tribunal: "Decision-making would be damaged."

Lord Baker pointed out that in local government, where the vast majority of planning decisions are made, advice to councils from officials is fully available to the public. The tribunal concluded that making the officials' advice public after the minister's decision would not "undermine to any significant extent the proper and effective performance by civil servants of their duties in future".

Mr Tomlinson said: "This case is very important for the planning process. It shows the process really does have to be open and transparent, that the information tribunal will take a robust attitude to government arguments that advice should remain secret, and that the Freedom of Information Act is actually working."

Lord Baker said he believed the act had held the government to account more than the House of Commons. "This will hold Mr Prescott to account as no other way could have done for an appalling decision which will have such a damaging effect on the London skyline." The department for communities and local government said it was studying the ruling.